Optical structures that scatter or diffuse light generally function in one of two ways: (a) as a surface diffuser utilizing surface roughness to refract or scatter light in a number of directions; or (b) as a bulk diffuser having flat surfaces and embedded light-scattering elements.
A diffuser of the former kind is normally utilized with its rough surface exposed to air, affording the largest possible difference in index of refraction between the material of the diffuser and the surrounding medium and, consequently, the largest angular spread for incident light. However, some prior art light diffusers of this type suffer from a major drawback: the need for air contact. The requirement that the rough surface must be in contact with air to operate properly may result in lower efficiency. If the input and output surfaces of the diffuser are both embedded inside another material, such as an adhesive for example, the light-dispersing ability of the diffuser may be reduced to an undesirable level.
In one version of the second type of diffuser, the bulk diffuser, small particles or spheres of a second refractive index are embedded within the primary material of the diffuser. In another version of the bulk diffuser, the refractive index of the material of the diffuser varies across the diffuser body, thus causing light passing through the material to be refracted or scattered at different points. Bulk diffusers also present some practical problems. If a high angular output distribution is sought, the diffuser will be generally thicker than a surface diffuser having the same optical scattering power. If however the bulk diffuser is made thin, a desirable property for most applications, the scattering ability of the diffuser may be too low.
Despite the foregoing difficulties, there are applications where a surface diffuser may be desirable, where the bulk type of diffuser would not be appropriate. For example, the surface diffuser can be applied to an existing film or substrate thus eliminating the need for a separate film. In the case of light management in a LCD, this increases efficiency by removing an interface (which causes reflection and lost light).
It is desirable to have the amount of diffusion vary across the diffusion film, whether a bulk or surface diffuser, to compensate for uneven brightness across a backlit display. A diffuser film with uniform diffusion across the film must have the diffusion efficiency to diffuse the most intense, specular areas of the display across the entire diffuser. These diffusers tend to need high levels of diffusion efficiency causing lower percentages total transmission across the entire film. With a diffuser with variable diffusion efficiency, the areas of high specular light could be diffused more than areas of less specular light. The result would be a display that had even diffuse light across it while having a higher overall transmission value compared to the uniform diffusion film.
The colored variable diffusion film evens out the color and illumination of the backlight across the display using a gradient in color tone, color density, and diffusivity. For a backlight with the light source in the center of the display, the percent total light transmission would increase and diffuse light transmission would decrease from the center of the roll to the edge of the roll. The film would be more diffuse and less transparent in the center of the display where the light is located, to compensate for the light intensity of the light bulb. Towards the edge of the film and display, away from the light source, more light passes through the film and the light is diffused less to create an even light intensity across the entire display. Selective coloration of the variable diffuser creates an even color tone across the display by using higher density color close to the light source and lower color density away from the light to create an even color across the display. Using colored variable diffusers even illumination and color across the display can be achieved.
In U.S. Pat. No. 6,270,697 (Meyers et al.), blur films are used to transmit infrared energy of a specific waveband using a repeating pattern of peak-and-valley features. While this does diffuse visible light, the periodic nature of the features is unacceptable for a backlight LC device because the pattern can be seen through the display device.
U.S. Pat. No. 6,266,476 (Shie et al.) discloses a microstructure on the surface of a polymer sheet for the diffusion of light. The microstructures are created by molding Fresnel lenses on the surface of a substrate to control the direction of light output from a light source so as to shape the light output into a desired distribution, pattern or envelope. The materials disclosed in U.S. Pat. No. 6,266,476 shape and collimate light, and therefore are not efficient diffusers of light particularly for liquid crystal display devices.
It is known to produce a base having a resin coated on one surface thereof with the resin having a surface texture. This kind of base is made by a thermoplastic embossing process in which raw (uncoated) base is coated with a molten resin, such as polyethylene. The base with the molten resin thereon is brought into contact with a chill roller having a surface pattern. Chilled water is pumped through the roller to extract heat from the resin, causing it to solidify and adhere to the base. During this process the surface texture on the chill roller's surface is embossed into the resin coated base. Thus, the surface pattern on the chill roller is critical to the surface produced in the resin on the coated base.
One known prior process for preparing chill rollers involves creating a main surface pattern using a mechanical engraving process. The engraving process has many limitations including misalignment causing tool lines in the surface, high price, and lengthy processing. Accordingly, it is desirable to not use mechanical engraving to manufacture chill rollers.
In U.S. Pat. No. 6,381,068 (Harada et al.) the diffusing element may be a bulk diffuser including a transparent base material of and at least one light-diffusing material, such as a pigment and/or beads, dispersed in the transparent base material. The pigments used may include a white pigment (for example, titanium oxide) and may also include one or more colored pigments. The pigments in this invention are only used with a uniform diffuser and not a variable diffuser. Furthermore, the colored pigments must be a single color tone and density across the display. The colored variable diffuser can tailor the color and diffusion properties of the film as a function of location on the film with varying colors and densities across the film.
In U.S. Pat. No. 6,266,476 (Shie et al.) a monolithic element having a substrate body and a macro-optical characteristic produced by surface micro-structures. These micro-structures can be non-uniform across the lens to minimize certain lens aberrations. These non-uniform micro-structures reduce lens aberrations, but are not able to significantly alter the macro-optical characteristics of the optical body. The diffusing structures, in this invention, vary as to change the macro diffusion efficiency across the diffusion film. The diffusion elements can vary changing the diffusion characteristics of the diffusion area from diffusing most of the light to letting light pass specularly which micro-structures are unable to do.
U.S. Pat. No. 6,285,001 (Fleming et al) relates to an exposure process using excimer laser ablation of substrates to improve the uniformity of repeating microstructures on an ablated substrate or to create three-dimensional microstructures on an ablated substrate. This method is difficult to apply to create a master chill roll to manufacture complex random three-dimensional structures and is also cost prohibitive.
In U.S. Pat. No. 6,124,974 (Burger) the substrates are made with lithographic processes. This lithography process is repeated for successive photomasks to generate a three-dimensional relief structure corresponding to the desired lenslet. This procedure to form a master to create three-dimensional features into a plastic film is time consuming and cost prohibitive.
U.S. Pat. No. 6,093,521 describes a photographic member comprising at least one photosensitive silver halide layer on the top of said member and at least one photosensitive silver halide layer on the bottom of said member, a polymer sheet comprising at least one layer of voided polyester polymer and at least one layer comprising nonvoided polyester polymer, wherein the imaging member has a percent transmission of between 38 and 42%. While the voided layer described in U.S. Pat. No. 6,093,521 does diffuse back illumination utilized in prior art light boxes used to illuminate static images, the percent transmission between 38 and 42% would not allow a enough light to reach an observers eye for a liquid crystal display. Typically, for liquid crystal display devices, back light diffusers must be capable of transmitting at least 65% and preferably at least 80% of the light incident on the diffuser.
In U.S. Pat. No. 6,030,756 (Bourdelais et al), a photographic element comprises a transparent polymer sheet, at least one layer of biaxially oriented polyolefin sheet and at least one image layer, wherein the polymer sheet has a stiffness of between 20 and 100 millinewtons, the biaxially oriented polyolefin sheet has a spectral transmission between 35% and 90%, and the biaxially oriented polyolefin sheet has a reflection density less than 65%. While the photographic element in U.S. Pat. No. 6,030,756 does separate the front silver halide from the back silver halide image, the voided polyolefin layer would diffuse too much light creating a dark liquid crystal display image. Further, the addition of white pigment to the sheet causes unacceptable scattering of the back light.
In U.S. Pat. No. 5,223,383 photographic elements containing reflective or diffusely transmissive supports are disclosed. While the materials and methods disclosed in this patent are suitable for reflective photographic products, the % light energy transmission (less than 40%) is not suitable for liquid crystal display as % light transmission less than 40% would unacceptable reduce the brightness of the LC device.
In U.S. Pat. No. 4,912,333, X-ray intensifying screens utilize microvoided polymer layers to create reflective lenslets for improvements in imaging speed and sharpness. While the materials disclosed in U.S. Pat. No. 4,912,333 are transmissive for X-ray energy, the materials have a very low visible light energy transmission that is unacceptable for LC devices.
In U.S. Pat. No. 6,177,153, oriented polymer film containing pores for expanding the viewing angle of light in a liquid crystal device is disclosed. The pores in U.S. Pat. No. 6,177,153 are created by stress fracturing solvent cast polymers during a secondary orientation. The aspect ratio of these materials, while shaping incident light, expanding the viewing angle, do not provide uniform diffusion of light and would cause uneven lighting of a liquid crystal formed image. Further, the disclosed method for creating voids results in void size and void distribution that would not allow for optimization of light diffusion and light transmission. In example 1 of this patent, the reported 90% transmission includes wavelengths between 400 and 1500 nm integrating the visible and invisible wavelengths, but the transmission at 500 nm is less that 30% of the incident light. Such values are unacceptable for any diffusion film useful for image display, such as a liquid crystal display.